The invention described herein relates to nuclear reactor fuel assemblies, and more particularly to an improved design of bulge tool used for securing a grid to control rod guide tubes which extend the length of a fuel assembly.
A nuclear reactor fuel assembly includes top and bottom support members, having a multiplicity of fuel rods and control rod guide tubes supported therebetween. Each fuel rod and control rod guide tube is separately held against lateral displacement by grids of egg crate configuration which are spaced along the fuel assembly length. Since the fuel rods and control guide tubes are made of Zircaloy and the grid used for supporting these components in spaced relationship with each other are made of Inconel, the incompatibility of these materials in a reactor requires that the grids be held in an exact position along with fuel assembly length by mechanical means, rather than brazing, welding or some other arrangement.
In one well-known grid design, short sleeves which correspond to the number of control rod guide tubes in the fuel assembly, are brazed at appropriate points in the cells formed by the interleaved grid straps. Each sleeve projects about 2 inches beyond each side of the grid. During assembly of a fuel assembly, the grids are mounted in predetermined positions and after the control rod guide tubes are pulled through the grid sleeves, a bulging tool is moved into the control rod guide tube and stopped at a point just below a grid strap, but still inside a sleeve which extends through a grid cell. The tool is then actuated to cause projections on the tool to plastically deform the control rod guide tube and sleeve. The bulging tool is then moved to a point just above the grid, and the process of plastically deforming the material again repeated with the result being that the grid is mechanically locked and rigidly secured to the control rod guide tubes in the fuel assembly.
The bulge tool of the prior art consists of a hollow cylinder having an outer diameter less than the inner diameter of a guide tube into which it was adapted to be inserted. The cylinder has four slots cut along its axial length, which forms four tines or ligaments. The wall at the outer end of each time was made of a greater thickness than the remainder of the tine length, and a circular opening was formed therein to accommodate a ball bearing ball. Since the wall of each tine was not sufficiently thick to capture the ball and hold it in place, the ball was placed manually in each tine just prior to placing the tool into use. A tapered ram or plunger was then inserted in the cylinder so that as the ram was forced toward the open end of the tines, the tapered surface engaged the balls and forced them radially outward into contact with the guide tube to produce spherical bulges in the sleeve and the control rod guide tube positioned therein.
The primary disadvantage of this design of tool is that the inner diameter of the tines or ligaments was held at a fixed diameter by the cylindrical surfaces on the ram. This fixed diameter was set in an effort to prevent the inner diameter of the guide tube, between bulges, from shrinking below an acceptable and preestablished limit. The reduction of circumferential length of the bulge also allowed a much larger inner diameter shrink-in due to the reduced chord diameter. This reduced chord diameter resulted from a drawing in or a contraction of the guide tube material appearing between adjacent bulges, and this action occurred at the time of making the bulges and the guide tube.
Another and perhaps more precise reason for such contraction or drawing in of the guide tube material between bulges, is that the slots or spaces between the tines on the cylinder, increased in size during the time bulges were being made. This increased space between tines provided an unsupported section of guide tube which allowed the material to distort to a flat shape rather than retaining its cylindrical configuration. It is apparent that the support in this area is very critical, because if the inner diameter shrinks or contracts to a distance less than the outer diameter of a control rod, it will be obvious that the control rod will not freely be movable in the control rod guide tube during the time of reactor operation.
Also, the design is such that the balls remained stationary while the tapered ram moved linearly into engagement therewith and thus forced them outwardly to create the bulge. This moving contact between the tapered ram and the balls produces very high hertz stresses at the point of contact between the ball bearing balls and the tapered ram. These high stresses, combined with the sliding action of the ram, tended to gouge the ram's surface which not only detracted from its efficiency but also placed a severe limit on its useful life.